Metallic and other powders are in wide use in the metalworking industry in forming and shaping processes and in forming composite materials by sintering a mixture of powders. Often these applications require powders which are preferably spherical and of small, uniform size, e.g. 50-500 microns. Conventionally, powders were made by chemical reduction of metallic salts, comminution of solid stock, gas jet disruption of a stream of liquid metal (gas atomisation) and water jet disruption of a stream of liquid metal (water atomisation), and by other mechanical means. For certain critical applications, these techniques rendered poor results in terms of purity, size, shape and uniformity. A more satisfactory technique is the rotating electrode process taught in U.S. Pat. Nos. 3,099,041 and 3,802,816. Typically in this process an atmospherically controlled chamber is provided within which is included a stationary electrode and an elongate, rotatable rod of material to be processed which acts as the second electrode. A motor rotates the rod as it is longitudinally advanced toward the first electrode. An arc struck between the electrodes consumes the rod material and rotation of the rod casts the melted material off by centrifugal force so that it forms a powder. The bar is consumed until only an unusable stub remains, whereupon a new bar may be inserted.
One problem that is associated with the previously described device is the tendency of the free end of the bar or consumable electrode to wobble as a result of the centrifugal force acting on it due to its own rotation. Furthermore, the bars are typically sixty inches long and leave a ten-inch stub, which effects an efficiency of only eighty-three and a third percent. A higher conversion rate is clearly desirable.
These bars are available in reasonably pure form in generally two to three inch diameters. However, recently the demand has been for ever higher purity of the powders. Unfortunately, higher purity bar stock is not readily available in the two or three inch diameter size but in much larger sizes, e.g. eight to twenty-five inch diameters, which are difficult to handle and process.